Look at your PAM stack and verify it's setup properly, verify the users she'll exists on the server in question, verify you don't have pamaccess.so blocking this user. These are just a few places to start.
I am trying to track down why one user all of a sudden can't log into a machine. He can use his LDAP passwd through our network, but gets an immediate login failure on a particular machine. I can 'su' to his account, from my account using his password, but when I try to log into this machine remotely, I get the failure and it terminates the sessions. There have been no changes to the system at all. I am looking for step to troubleshoot this issue. Thanks.